Process of beneficiating cherty minerals



Patented Dec. 13, 1949 PROCESS OF BENE MIN FIOIATIN G CHERTY ERALS Harold Dunton, San Jose, Calif.

No Drawing. Application July 24, 1945, Serial No. 6965882 This invention relates to the beneficiating of chert and cherty minerals, and it has particular reference to the beneficiating of cherty 1ime- 6 Claims. (01. 23-182)- flint occurring in large masses in rocks of the carbonifer-ous system and also as disseminated grains, chiefly in limestone rocks. Chent is ve y diflicult to grind. This is the reason why no commercial use of chert and cherty rocks has beenpossible hitherto, except those occasions in which slabs or blocks of the natural stone can be employed. Where the limestone itself was of a high degree of :purity, hand picking on belts has beenpracticed to separate the CaCO3 from large veins,- nodules, or masses of chert irregularly dispersed within the stone; the chert picked out was re- .Iected.

In view of this situation it is among the objects of the present invention to provide-a. method for converting chert into a high grade siliceous material that is useful for many commercial purposes; moreover to recover chert converted in 1 this manner from cherty minerals of high chert content. And it is a particular object of the invention to provide a method for recovering high grade calcium oxide from cherty limestones While at the same time causing the chert lenses which appear embedded in such limestones to be transformed into a valuable commercial silica source.

This invention is based on the discover that by a heat treatment with subsequent chilling or quenching the physical state of chert is changed from one of extreme hardness to one of such softness that the material can be easily ground to any degree of fineness required for commercial purposes, such as for making certain grades of glass, e. g. bottle glass, in the manufacture of refractories and ceramics, of stuccos or plasters consisting of asphaltic mixtures, of wall boards, shingles, in preparing certain grades of concrete (limited by its softness), and in many other uses. It has been found that for the purpose of effecting the transformation aimed at, the'burning is to be performed to temperatures above 1000 C., advantageously around 1150 C. or even somewhat higher; chilling may be effected by contacting the hot material with a gaseous chilling agent or by treating it with, or immersing it in, water while still hot.

In applying that latter method to cherty limestones, in the \calcination stage the calcium carbonate isconverted into quicklime which is then hydrated and disintegrated into an extremely or will form a slurry if suflicient water is added. 0n the other hand, the silica in spite of being materially changed in its physical condition remains'in practically the original condition as to size and shape. The calcium hydroxide can then" easily be separated from the chert either by sizes' or by density or in both ways. In this manner two commercially valuable products are enabled to -'be recovered from natural cherty limestones which are of little or no-practical use in their original state.

It has been known for a long time to use calcination with subsequent hydration and mechanical separation for recovering high grade lime from calcareous minerals carrying accompanying substances of a different mineralogical nature, whether said foreign concomitantswere used in themselves or rejected.

As early as in 1891 the suggestion was made in the U. S. Patent No. 453,753 to Baude to prepare pure lime for the manufacture of Portland cement from blue limestone, a formation known to geologists as the "Hudson River or Cincinnati group, by burning the limestone as it is ordinarily done in the process of making quicklime, and placing the lumps ofcalcined limestone as they come fromthe kiln in wire baskets, immersing them in water, and then quickly removing them. On emptying out the lime upon a floor and allowing it to remain until it is slaked, only the :pure lime will slaken, that is to say be-' come hydrated, and thereby be caused to crumble, whereas the foreign matter and the impure lime-that is lime containing silicawill not slakemhut remain in lumps and can then be sepanated from the hard solid particles by sifting; A similar proposal is made in the British specification No. 1,110-1908 to Buendia in connection with the treatment of ores in general for the purpose of recovering pure quicklime on the one hand and enriched ore on the other, or with a view of purifying lime.

This principle of celebration with subsequent hydration and separation has later on repeatedly been used with slight modifications in the =bene- Theevomtionof carbon dioxide from CaCOa':

begins between 600? C. and 725 C. and continues 3 up to 850 C. to sow C. though when large masses are being burned for the production of quicklime the kilns are usually heated to 900 C. to 1200 C. (1652 F. to 2192 R), the maximum temperature depending on that at whichthe impurities fuse and so cause the lime to become overburned (cf. Alfred B. Searle, Encyclopaedia of the Ceramic Industries, Ernst Benn Ltd., London, vol. II, 193Qypage 2577-. Yet, to change the physi'-- cal conditions of the chert, the chertylimestone must necessarily be burned to unusually high temperatures. Of course, also in the present process the temperatures are conditioned by. the

degree of fineness of the material to be treated and the duration of the calcining treatment;

yet as a rule the required temperatures come near or even exceed the uppermost limit per-- missible in producing quicklime from purest cal-u careous rocks. Those temperatures do not cause combination between the 021003 and silica or alumina during the calcination, and as a con sequence thereof during the subsequent hydra-v tion, as would be thecase in an-argillaceous limes stone; cherty limestones; behave otherwise hecause-in such limestones thesilica is'in: close contact with CaCOs only at the interbedding faces between the separate layerswof SiQzand CaGOa; Funthermore, in chertylimestones both the S102 and: GaCOs are substantially free from Fe and Al. even athigher burning: temperatures than are usual in producing quicklime from high'igrade calcites; reactionsbetweena the lime and the silica and alumina, formingsilicatesz and aluminates.

which would interfere with; the: subsequent sep.-

analtion; process do. not. occurs but: 602a, very; lim;. ited extent.

R garding. this-detail, attention; is t-ovbe called to U. S. Patent No. 1,150,8971to;Spencer;.though thedisclosure as a; whole is; not pertinent to. the consideration in hand. This specification describes as an alternative a method of operation; in which the calcination of: calcareous zinc. tailings; is; performed at a temperature high enough not only td-produce quicklime from the calcare cuss ganguebut to. convert: it into am overb rnti form that reacts but; very slowly; or not: at all; with; water (page; 2,, lines, 38 to. 53)., this; the over-burnt calcareous angue, forms: granules that can easily beseparated from the zinclmineralsr What is interesting'here, is the.

temperature contemplated; for such an overburm lngof: the lime. In this respect, on page 2, the following is stated in lines; 1131510118:

In the calcination, any temperature above700fi C. may heemployed though it is desirablaand particularly inproducin overburning, towork at. a rather'higher range, say for, producing. quick,- limei. around 95,096: and for'overburning, around 10.5.0 0,." (1922 F;)

It. is; anindispensable requirement. to success;

This is the reason-why in. chertyalimest'onea :c

In: doing that contain all those constituents, but in unsuit' able proportions. In: this case it iso'bviouslya 75 ess the-cherty limestones have necessarily to be;

necessity to use argillaceous limestones which contain the required amount of alumina (or an excess thereof) besides CaCOs. However, to render IthiS possible, the original :principle had to be modified in a manner opposite to the modification necessary for making the original method applicable to the treatment of chert limestones.

According to Breerwood the crushed product is calcined; to a degree sufficient to obtain a substantially complete ignition loss, but without heating the rock for a sufiicient length of time and at a temperature by which material reac- 1510 18. between the calcium and the silica and to some extent the alumina would be caused to take place during oalcination, and which would pantieularly favor the occurring of such premature re actions, in: tlhe: course of the subsequent hydrathan. This condition is stated to be slightly critlcal, owing to the presence of combinable silica,

which is" made more reactive by the heating process. Therefore the step of oalcination should not beiconfused'wwith the-calcination or'high grade limestones; for thexpm'llosaof producing caustic: lime? (page 2; 1: 0., last paragraph: and r. c.,lines Astoz the hydra ion; this second stephas tobe efiected according to: Breerwood by stirring the" med-iately:; and. the argillaceous: compounds; very.-

quicklythereafter; usually within about; 45-secend-s, whereas the calcium hydrate formed? re.-

\ mains: suspended. The surplus of that precipittated constituent which isin excessiis then discarded: and: the remainder returned to the, other merged t is stated-that this reaction may be so complete; due, to excessive heat treatment, that the; rock will not disintegrate. therefrom that the degree of separation is limited; by. the extent. of these reactions, As the com.- pounds formed, by; premature reactions are elimzinatedfrom. the;,mixture as waste, it'follows that controlling calcinaticn conditions such that premature reactions are prevented from taking place,

is alsoiessential. in the economical employment of It. will be seen theprocess (page 2, r. 0., lines 31 to 4-2"). The requirediconditions are exemplified in the specificationby the statement (of. page 4, l-. c., linesl'? to 24); that a. rock of. three-quarter inch ring size,.

when calcined: at a temperature of 1890 F; for" approximately one hour and 50 minutes will dis-v integrate within; three minutes i even without agitation, after submergencein ten to fifteen. vol

times of water, with, relatively; little lossv due to premature reactions.

It may be emphasized again that in. order to change the physicalstate of the chert from one of extreme hardness that makes it difil'cult 'to grlndto one of great softness, in the present procw I calcined at temperatures that are, even higher than usual in the calcination of-high grade limestones for the purpose of producing caustic lime. And this can be done, as has been set forth above in detail, because those temperatures when applied to cherty limestones do not cause combination of the lime with the silica and with the alu-' mina, seeing that in cherty limestones the silica is in close contact with CaCOs only at the interbedding faces between separate layers, and the said limestones are substantially free from al-umina. As a consequence thereof, even in effecting the calcination at temperatures that are higher than the maximum temperatures admissi ble in producing caustic limes, with cherty limestones there is no risk of silicates being formed in an amount that would interfere with, or even prevent, the subsequent separationof the silica from the lime. This is the reason why this invention does not apply to argillaceous limestones, but to siliceous limestones which-contain but small percentages of sesquioxides and are practically free from clay material, and applies particularly to those materials or deposits of limestones in which the limestone component is above 90% CaCOs, and the silica component, referring to chert only, is above 95% S102. In argillaceous limestones carrying finely divided silica, iron, and alumina, very large surfaces would be exposed to the chemical action of the lime, which action sets in, at the temperatures cogently required in the present process, as soon as quicklime is produced through calcination in any amount whatever. Also the calcium hydroxide formed by the subsequent hydration were bound to combine largely with silica, alumina, and iron.

As an example of a cherty limestone which may advantageously be treated in accordance with the invention the following analysis is given:

The lime material and the chert material recovered from cherty limestone analyses in the calcined state as follows:

Ohert Material Lime Material In carrying the process into practice the chert or cherty rock is crushed to sizes workable in a rotary kiln. The rock is preferably crushed to about one-half inch to one and a half inch and the undersize removed, as this increases the calcining capacity of the kiln, and enhances in the treatment of cherty limestone, the subsequent separation of the silica and calcium oxide. The process calls for a calcination temperature of i2200 F. (1204.4 C.) as the most favorable, but will vary within reasonable limits depending on the nature and sizes of the original rocks. After calcination for a period of about one-half hour the material is chilled by cooling it rapidly with a blast of air or by sprinkling it with, or, immersing it in, water while still hot. In case of treating cherty limestones only the latter method is applicable. Under the action of water. in the calcined mixture that consists of substantially pure S102 and substantially pure CaO, therCaO converts to Ca(OH) 2, whereby the lumps disintegrate into a very finely divided powder, whereas the lumps of silica arenot affected by this action as to their form and size. On the other hand the physical state of the silica changes from practical ungrindability to such adegree of softness that the material can easily be groundto any degree of fineness for commercial use. The coarse S102 and fine Ca(OH)2 particles are separated from each other by sifting or by mechanical classification, as with the aid of a bowl classifier, or by combining both these methods with each other. According to a preferred embodiment of the method, enough water is added to the hot cal-' cined material to permit its continuous removal in a semi-liquid state, whereupon separation of the mixture is effected by passing it over a vibrating screen, the oversize of which is being brought in a suitable washing device for the pur pose of removing adhering calcium hydroxide from the chert surfaces. The lime washed out may then be thickened in any suitable form of thickener and the thickened calcium hydroxide added to that recovered through the treatment on the vibrating screen, or reserved for separate use.

Alternatively, the calcined cherty rock is caused to drop directly in a bowl classifier, overflowing the high grade hydroxide from the bowl and removing the coarse chert by the rakes, whereupon the hydroxide is thickened in standard thickening equipment and the two products are further treated to adapt them to particular purposes.

In any case it will be recommendable to subject the washed chert to the action of acids and to wash out the so treated material with water for eliminating the remnants of acid.

It was hitherto the practice to previously calcine chert for the purpose of adding it to a major amount of saggar clay. Yet this was done without subsequent chilling or quenching and only for the purpose of lessening the tendency of the material to spall.

In addition to the advantages of the present method as set forth above, it may be mentioned that a high grade lime results therefrom which thickens relatively readily owing to the high temperatures employed in calcination. As a consequence thereof the hydroxide can easily be separated from the bulk of the water added for the purpose of hydration, and from wash water, if desired, even in repeated operations. The lime prepared in this way is pure enough for use in the manufacture of Portland cement and the like.

What is claimed is:

1. A process of beneficiating cherty minerals containing a predominant proportion of chert by weight to obtain soft high grade silica which comprises heating the material to be treated to a calcining temperature of from about 1000 C. to

about 1200 C. thereby converting the silica from a state of extreme hardness to one of such softness that it can be easily ground, rapidly cooling the calcined material, and separately recovering the silica component.

2. A process according to claim 1 in which the material is cooled by immersion in water while substantially at calcination temperatures.

3. A process according to claim 1 in which the material is cooled with a gaseous chilling medium.

4. A process for producing hydrated lime and high-grade silica of such softness that it may be easily ground from substantially clay-free highly siliceous limestones containing silica in the form mammal:

of cher't inapropor tion's approaching "the amount of lime in the form of calcium carbonate which comprises crushing the cherty limestone to a predeterminefi size, removing unders'ize material, calcining:said crushed, sizedcher-ty limestone at a temperatureofiromab'cut i000 C. toabout 1200 C.,'*immersing the material in water while subs'tantially "at *calcination "temperatures to hydrate and disintegrate the lime component "thereby :5. A process faccordin'g-tolclaim .4 in which the limestone contains :silica in the form of cher'tsanfi Iimein the formo'fcarbonatein about-equal parts by weight, and in which the lime an-d modifieii silica arezseparated by -sifting.

6. A :process according to-olaim 4 in which the limestone con'tains siiica in the form df-chert and 1 lime in the firm or vcarbonate in about equal parts byweigh't, and "in which the *lime andmedified s'ilica. are separated by hydraulic classifications.

HAROISD J. DUNTON.

The "icilowin'g "references are vcf record in the file or this yatent:

Nuniber Name Date 1;634;424 iiiunter July 5, 1827 FOREIGN .PATENTS Number Country 'Date Great Britain Jan. 25, 1 924- 

